Catalysts are sieved before they are packed into drums to be shipped to their destination. Nevertheless, some attrition occurs in transit, particularly if the drums are roughly handled. As a consequence, some form of screening is usual before charging the catalyst to the reactor vessels. One method of screening which has been utilized in the prior art is to pass the catalyst over an inclined chute made of a suitably-sized mesh. Other methods have included passing the catalyst over vibrating screens. The former method quite often is more satisfactory since vibrating screens can cause further unnecessary fragmentation and loss of catalyst. Another useful method is to blow the dust away with a compressed air jet while the catalyst is pouring into a charging bucket or hopper. Simple hand sieving may be used. However, the operators must stop sieving when the dust has been removed, so that unnecessary attrition is avoided.
In charging the catalyst into the reactor vessels, the catalyst should not have a free fall of more than two to three feet and the catalyst must be distributed evenly as the catalyst bed is filled. The catalyst pellets should not all be poured into the vessel in one spot and subsequently raked level. When this is done, the particles tend to segregate, with small particles and dust remaining in the center of the heap, while the larger pieces roll to the edge. Uneven packing and breakage during charging seriously affects the gas distribution utilization of the catalyst. The degree of packing in the catalyst bed has a marked effect on the voidage so that in a bed of regularsize pellets, the voidage can vary by over .+-.10%, according to the degree of packing. If the granules are not all the same size or if fines or undersized catalyst pieces are present, the voidage variations can be considerably greater. The effect of packing on pressure drop is even more marked because the pressure drop is roughly proportional to voidage. Thus, the difference in the pressure drop between loosely packed and closely packed beds of regularsize pellets is roughly in the ratio of about 1:2. In practice, the pressure drop increases by about 50% when the catalyst bed settles during use.